OVERVIEW

The reason for putting this chapter before other advanced interpretation techniques is that I believe it is of primary importance in correctly defining the STOIIP (stock tank oil initially in place) or GIIP (gas initially in place) of a field. Indeed, because of the way that dynamic reservoir models are constructed in many fields in practice, it completely supersedes any exotic models constructed by the petrophysicist for calculating saturations.

Many times in my career I have seen petrophysical departments working in isolation constructing fabulously complicated models to calculate saturations. But when you ask the geologist what saturations have gone into the static model, he will tell you that he is using a constant value unrelated to the zonal weighted averages. The reservoir engineer may be using one P _c/ S _w table in the simulator based on just one air/mercury capillary pressure measurement that he felt was representative of the reservoir in general.

I believe perhaps the most important role the petrophysicist has in a petroleum engineering department concerns ensuring that the saturation/height function being used in the static and dynamic models represent the best possible combination of core and log data, combined with sound petrophysical judgment. In my view, such a function should have both porosity and permeability as input variables, together with height (which may be directly related to P _c).

There are dozens of different functions that have been used to describe capillary behavior in rocks. I have used many of these over...